Certain forms of human dental decay are associated with elevated levels or proportions of Streptococcus mutans. We are testing the hypothesis that S. mutans is responsible for a measurable amount of this decay. One approach involves longitudinal studies in which the levels and proportions of S. mutans on caries free fissures are being monitored at approximately 6 month intervals in order to determine whether S. mutans colonization and proliferation predisposes to caries in that fissure. In separate longitudinal studies, the acquisition of S. mutans by infants and the intraoral spread of S. mutans in adults is being investigated. In a second clinical approach, rampant caries individuals, i.e. patients with greater than or equal to 10 carious tooth surfaces, are being treated by private practitioners in their own offices using a protocol which includes short-term antimicrobial treatment in addition to the normal restorative procedures required by these patients. All patients receive the restorative treatment but only half receive the antimicrobial treatment and the other half receive a placebo treatment. Children 5 to 6 years of age, without any erupted permanent molars are treated either with 1.2% neutral NaF or a placebo gel. The gels are administered in applicator trays and are applied by the patients at home twice a day for a 1-week period at approximately six month intervals. Fissure and proximal plaque samples and saliva are cultured before and after each gel treatment period for total viable counts, S. mutans and S. sanguis counts.